In cellular radio networks, a base station is needed to be placed in each region that requires coverage. Prior deployment strategies assumed a regular cell topology, resulting in the emplacement of radio base stations according to a strict geometric pattern. However, in real-world deployments, identically-sized cells are ill-suited to providing effective coverage because of topological features (i.e., mountains, hills, highways, etc.), and because of varying population density patterns, among other reasons.
To handle these varying characteristics, strategies involving multiple cell sizes in a heterogeneous network, or hetnet, have been proposed. For example, a traditional macro cell base station may be used to cover a relatively large area, but may be supplemented in an area of increased population density by a cell with a smaller coverage area. Such cells are variously called femto cells, pico cells, micro cells, or generically, small cells. The term femto cell is used in this disclosure to refer to one or more of these various types of cells.
However, integration of these base stations of various sizes causes interference between cells. This is particularly true because femto cells are often placed in a location that overlaps substantially or completely with the coverage area of a macro cell, e.g., in overlay or underlay coverage patterns. When a macro cell coverage area overlaps completely with a femto cell coverage area, without mitigation techniques, the femto cell base station and the macro cell base station necessarily ends up competing for radio resources and reducing the effectiveness of attached mobile nodes via inter-cell interference.
Additional challenges with small cell deployments include: little or no RF planning; cell coverage areas that are not well-defined or that do not conform to a standard circular or hexagonal cell shape; the requirement to coexist with existing macro cells; small radius for small cells, making soft frequency reuse less than straightforward; and the necessity of central coordination when using UE-centric resource allocation.